The present invention is generally directed to a method and apparatus for controlling the flow of fluid from a source to a patient and removal of fluids from the patient through a phacoemulsification handpiece as well as controlling power provided to the phacoemulsification handpiece.
The flow of fluid to and from a patient through a fluid infusion or extraction system and power control to a phacoemulsification handpiece is critical to the procedure being performed.
A number of medically recognized techniques has been utilized for lens removal and among these, a popular technique is phacoemulsification, irrigation and aspiration. This method includes the making of a corneal incision, and the insertion of a handheld surgical implement which includes a needle which is ultrasonically driven in order to emulsify the eye lens. Simultaneously with this emulsification, a fluid is inserted for irrigation of the emulsified lens and a vacuum provided for aspiration of the emulsified lens and inserted fluids.
Currently available phacoemulsification systems include a variable speed peristaltic pump, a vacuum sensor, an adjustable source of ultrasonic power and a programmable microprocessor with operator-selected presets for controlling aspiration rate, vacuum and ultrasonic power levels.
Many surgical instruments and controls in use today linearly control the vacuum or linearly control the flow of aspiration fluid. This feature allows the surgeon to precisely xe2x80x9cdispensexe2x80x9d or control the xe2x80x9cspeedxe2x80x9d at which he/she employs, either the vacuum or the flow, but not both. However, there often are times during surgery when the precise control of one of the variables (vacuum, aspiration rate, or ultrasonic power) is desired over the other. The experienced user, understanding the relationship between the vacuum and the flow, may manually adjust the preset variable appropriately at the console in order to obtain an acceptable performance. However, if this adjustment is overlooked, then the combination of both high vacuum and high flow can cause undesirable fluidic surges at the surgical site with possible damage inflicted on the patient.
It should be apparent that the control of handheld surgical instruments for use in phaco surgery is complex. Phacoemulsifier apparatus typically comprises a cabinet, including a power supply, peristaltic pump, electronic and associated hardware, and a connected, multi-function and handheld surgical implement, or handpiece, including a hollow slender-like needle tube as hereinabove described, in order to perform the phacoemulsification of the cataractous lens.
It should be appreciated that a surgeon utilizing the handheld implement to perform the functions hereinabove described requires easy and accessible control of these functions, as well as the ability to selectively shift or switch between at least some of the functions (for example, irrigation and irrigation plus aspiration) as may arise during phacoemulsification surgery.
In view of the difficulty with adjusting cabinet-mounted controls, while operating an associated handheld medical implement, control systems have been developed such as described in U.S. Pat. No. 4,983,901. This patent is to be incorporated entirely into the present application, including all specification and drawings for the purpose of providing a background to the complex controls required in phacoemulsification surgery and for describing apparatus which may be utilized or modified for use with the method of the present invention.
To further illustrate the complexity of the control system, reference is also made to U.S. Pat. No. 5,268,624. This patent application is to be incorporated in the present application by this specific reference thereto, including all specifications and drawings for the purpose of further describing the state-of-the-art in the field of this invention.
It should thus be apparent, in view of the complex nature of the control system of fluids and ultrasonic power in the case of phacoemulsification procedures, that it is desirable for a surgeon to have a system which is programmable to serve both the needs of the surgical procedure and particular techniques of the surgeon, which may differ depending on the experience and ability of the surgeon.
The present invention more specifically relates to power control to a phacoemulsification handpiece based on the determination of the phase angle between voltage applied to a handpiece piezoelectric transducer and the current drawn by the piezoelectric transducer and/or the amplitude of power pulses provided to the handpiece.
Phacoemulsification systems typically include a handpiece having an ultrasonically vibrated hollow needle and an electronic control therefor.
As is well known in the art, the phacoemulsification handpiece is interconnected with a control console by an electric cable for powering and controlling the piezoelectric transducer and tubing for providing irrigation fluid to the eye and withdrawing aspiration fluid from an eye through the handpiece.
The hollow needle of the handpiece is typically driven or excited along its longitudinal axis by the piezoelectric effect in crystals created by an AC voltage applied thereto. The motion of the driven crystal is amplified by a mechanically resonant system within the handpiece, such that the motion of the needle connected thereto is directly dependent upon the frequency at which the crystal is driven, with a maximum motion occurring at a resonant frequency.
The resonant frequency is dependent, in part upon the mass of the needle interconnected therewith, which is vibrated by the crystal.
For pure capacitive circuits, there is a 90 degree phase angle between a sine wave representing the voltage applied to the handpiece and the resultant current into the handpiece. This is expressed by the angle equaling xe2x88x9290 degrees. For a purely inductive circuit, the phase angle equals +90 degrees and, of course, for purely resistive circuits=0.
A typical range of frequency used for phacoemulsification handpiece is between about 30 kHz to about 50 kHz. A frequency window exists for each phacoemulsification handpiece that can be characterized by the handpiece impedance and phase.
This frequency window is bounded by an upper frequency and a lower cutoff frequency. The center of this window is typically defined as the point where the handpiece electrical phase reaches a maximum value.
At frequencies outside of this window, the electrical phase of the handpiece is equal to xe2x88x9290 degrees.
Handpiece power transfer efficiency is given by the formula (V*I)(COS). This means that the most efficient handpiece operating point occurs when the phase is closest to 0 degrees.
In order to maintain optimum handpiece power transfer efficiency, it is important to control the frequency to achieve a phase value as close to zero degrees as possible.
This goal is complicated by the fact that the phase angle of the ultrasonic handpiece is also dependent on the loading of the transducer which occurs through the mechanically resonant system which includes the needle.
That is, contact by the needle with tissue and fluids within the eye create a load on the piezoelectric crystals with concomitant change in the operating phase angle.
Consequently, it is important to determine and measure the phase angles at all times during operation of the handpiece in order to adjust the driving circuitry to achieve an optimum phase angle in order to effect constant energy transfer into the tissue by the phaco handpiece, regardless of loading effects.
Thus, it is important to provide automatic tuning of the handpiece during its use in phacoemulsification tissue and withdrawing same from an eye. This auto tuning is accomplished by monitoring the handpiece electrical signals and adjusting the frequency to maintain consistency with selected parameters.
In any event, control circuitry for phacoemulsification handpiece can include circuitry for measuring the phase between the voltage and the current, typically identified as a phase detector. However, problems arise in the measurement of the phase shift without dependence on the operating frequency of the phacoemulsification handpiece. That is, because, as hereinabove noted, the phase shift is dependent on the operating frequency of the handpiece and time delay in the measurement thereof requires complex calibration circuitry in order to compensate to provide for responsive tuning of the handpiece.
Phase detection is the process of applying two electrical periodic signals of similar frequency into an electrical circuit that generates a voltage proportional to the time (phase) difference between the two signals.
This voltage generated by the phase detector is then usually time averaged either by an electronic circuit or sampled by an A/D converter and then averaged digitally.
The averaged signal can be read by a conventional voltage meter or used by a microprocessor as date for processing. The averaging also helps to reject electrical noise.
As was described earlier, the output of a phase detector is proportional to the difference in time (of occurrence) to two signals. By definition, this means that while the electrical output of a conventional phase detector is a function of the signal phase, it is also directly proportional to the frequency of use. This means that the frequency of use must be known and compensated for when reading the phase detector output in order to derive quantified phase values. While, as hereinabove noted, a calibration circuit can account for the variation of the frequency, such a circuit is usually very complex and may require the use of a microcontroller. In addition, neither of these approaches account for the drift in performance over time which is typical of phacoemulsification handpieces.
This problem was recognized in U.S. Pat. No. 5,431,664, which provided a solution by using the admittance of the transducers as the tuning parameter rather than the phase-angle. The necessary circuitry is, of course, complicated and accordingly there is still a continuing need for a method for determining real time electrical phase for a piezoelectric phacoemulsification handpiece which is consistent over the entire handpiece range of operation which does not require further calibration circuitry for the controller.
The ultrasonically driven needle in a phaco handpiece becomes warm during use and such generated heat is for the most part dissipated by the irrigation/aspiration fluids passing through the needle. However, care must be taken to avoid overheating of eye tissue during phacoemulsification.
Interrupted power pulse methods have been developed in order to drive the needle with reduced heating to avoid overheating and burning of tissue. The present invention improves this power pulse method.
In accordance with the present invention, phacoemulsification apparatus generally includes a phacoemulsification handpiece having a needle and an electrical means for ultrasonically vibrating the needle. The power source provides a means for supplying pulsed electrical power to the handpiece electrical means and a means for providing irrigation to the eye and aspirating fluid from the handpiece needle is also incorporated in the present invention.
Input means is provided for enabling a surgeon to select an amplitude of the electrical pulse. Control means is provided for controlling a pulse duty cycle. In that regard, a controlled off duty cycle is established by the control means in order to ensure heat dissipation before a subsequent pulse is activated. Preferably the control means provides a pulse of less than 20 milliseconds or a repetition rate of between about 25 and about 2000 pulses per second.
In another embodiment of the present invention, a means for determining the voltage current phase relationship of the provided electrical power is provided.
In this embodiment, the control means is responsive to both the pulse amplitude and the determined voltage current phase relationship for varying a pulse duty cycle of the power supply to the handpiece.
The means for determining the voltage current phase relationship generally includes the means for obtaining an AC voltage signal corresponding to the operating AC voltage of a piezoelectric handpiece and means for obtaining an AC current signal corresponding to the operating AC current of the piezoelectric handpiece.
Means are provided for determining the onset of a current cycle from the AC current signal and means are also provided for producing a voltage (VI) corresponding to a time necessary for the AC current to reach a maximum value after onset of the current cycle.
Additionally, means are provided for producing a voltage (Vv) corresponding to a time necessary for the AC voltage to reach a maximum value after onset of the current cycle.
An A/D converter provides a means for comparing (Vv) and (VI) to determine the phase relationship between the voltage and current of the piezoelectric phacoemulsification handpiece and generating a phase signal (Sp) corresponding thereto, the phase signal being frequency independent.
A method in accordance with the present invention for operating a phacoemulsification system which includes a phacoemulsification handpiece, and an ultrasonic power source, a vacuum source, a source of irrigating fluid, and a control unit having a vacuum sensor for controlling the aspiration of the irrigating fluid from the handpiece. The method includes the steps of placing the handpiece in an operative relationship with an eye for phacoemulsification procedure and supplying irrigation fluid from the irrigation fluid source into the eye.
Pulsed ultrasonic power is provided from the ultrasonic power source to the handpiece for performing the phacoemulsification procedure. Preferably the pulsed power of a duration of less than 20 milliseconds or at a repetition rate of between 25 and about 2000 pulses per second.
A vacuum is applied from the vacuum source to the handpiece to aspirate the irrigating fluid from the eye through the handpiece at a selected rate.
An input is provided enabling manual selection of power pulse amplitude.
A voltage current phase relationship of the power from the power source may be determined and in response thereto, the ultrasonic power being provided to the handpiece is variably controlled.
In one embodiment of the present invention, the variable control of the power includes varying the pulse duty cycle of the supply power in response to the pulse amplitude and/or voltage current phase relationship.